Magnetic resonance imaging (MRI) is currently contra-indicated for patients who have implanted medical leads. This is due largely to the patient safety issue that results when the strong electromagnetic fields of an MRI system interact with the antenna-like therapy delivery leads of an active implantable medical device (AIMD). It is well documented that the radio frequency (RF) signals that are generated by the MRI system can couple along the length of a lead conductor or conductors and create induced RF currents. These RF currents can cause significant heating at points of high current concentration, the most significant of which is at the distal tip electrode, where the lead system makes direct contact with body tissue. Excessive RF currents at the point of electrode contact to tissue can create a serious or even life-threatening situation. In the literature, there are reports of MRI induced thermal damage to brain tissue from around the area of implanted deep brain stimulation electrodes, pacemaker pacing capture threshold changes or even loss of capture which means that a pacemaker dependent patient is without life support.
AIMDs can include completely implantable systems or a combination of externally worn devices with implanted leads. AIMDs include the group of cochlear implants, piezoelectric sound bridge transducers and the like. AIMDs can also include a variety of neurostimulators and brain stimulators. Sometimes these are called neuromodulators. Neurostimulators are used to stimulate the vagus nerve, for example, to relieve epilepsy, obesity, and depression. Deep brain stimulators are pacemaker-like devices and include electrodes implanted deep into the brain matter for sensing the onset of the seizure and also providing electrical stimulation to brain tissue to prevent the seizure from actually occurring. There are also other types of deep brain stimulators used to correct other abnormalities, such as Tourette's Syndrome and the like. AIMDs also include all types of cardiac pacemakers, left ventricular assist devices (LVADs), artificial hearts and implantable cardioverter defibrillators. AIMDs also include drug pumps which can be used for dispensing of insulin, chemotherapy drugs, pain medications and the like. AIMDs can include a variety of implanted or external-implanted bone growth stimulators for rapid healing of fractures. AIMDs may also include urinary incontinence devices, pain release spinal cord stimulators, anti-tremor stimulators, congestive heart failure devices, cardiac resynchronization therapy devices (CRT), and the like.
As disclosed in US 2007/0112398 A1 and U.S. Pat. No. 7,853,325, the contents of which are incorporated herein by reference, a novel method to minimize the expected heating at the distal tip of the lead system is to incorporate a bandstop filter. This bandstop filter is comprised of an inductor and capacitor in parallel, with the bandstop filter connected in series with one or more conductors of the implanted lead system. In such a system, the bandstop filter is constructed so that its resonant frequency or frequencies coincides with the RF operating frequency of one or more MRI systems.
RF frequencies are directly related to the MRI machine static magnetic field by the Lamour Relationship wherein the frequency is equal to 42.56 times the static field strength in Teslas (for hydrogen scanners). Typical MRI RF pulsed frequencies are 64 MHz for 1.5 T systems and 128 MHz for 3.0 T systems. At resonance, the impedance of the bandstop filter is quite high (for example, above 2,000 ohms) which reduces the flow of distal electrode to tissue current at the MRI RF pulsed frequency thereby reducing implanted lead and/or electrode heating. Increasing the impedance at the lead distal tip electrode greatly reduces the amount of RF current that would flow into body tissue. It has been documented that excess current can cause tissue damage, pacing capture threshold (PCT) changes or even tissue necrosis.
Implementation of this technology in implantable leads is a significant challenge. Bandstop filters for use in implantable lead systems must be biocompatible, not significantly change the electrical performance characteristics of the lead (except within the context of the invention), and must not significantly affect size, weight, or implantability. With increasingly smaller leads being developed to accommodate small vasculature and left ventricular pacing through the coronary sinus, bandstop technology must be equally scalable to match the same demands.
The present invention satisfies these needs and provides other related advantages.